Preclinical Pharmacokinetics, Tissue Distribution and in vitro Metabolism of FHND6091, a Novel Oral Proteasome Inhibitor

Introduction: FHND6091, a novel N-capped dipeptidyl boronic acid proteasome inhibitor with promising pharmacological proper-ties, entirely converted into active form FHND6081 under physiological conditions. The proteasome, a key component of the ubiquitin-proteasome pathway (UPP), has emerged as a validated target of multiple myeloma (MM) therapeutics. FHND6091 is a selective oral proteasome inhibitor that binds irreversibly to the beta 5 submit of the 20S proteasome and exerts anti-cancer roles.Methods: In this study, we investigated the metabolic stability, metabolite production, metabolic pathways and plasma protein binding (PPB) of FHND6081 along with its absorption, tissue distribution, excretion (ADME) and pharmacokinetics (PK) in animals. Results: Ultra-high performance liquid chromatography-tandem quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) identified a total of nine new metabolites after co-incubation with FHND6091 in hepatocytes from different species. A hypothetical CYP450-metabolic pathway including dehydrogenation, N-dealkylation plus mono-oxygenation and other was proposed. In addition, FHND6081 was highly bound to plasma proteins (>99%); nevertheless, it preferred to partition to red blood cells (B/P ratio: 4.91). The results of microsomal metabolic stability corroborated that FHND6081 was a moderate-clearance compound. In Caco-2 cell experiments, the compound displayed modest permeability suggesting that it may show limited bioavailability via oral routes. Furthermore, FHND6081 was extensively distributed in rats and the highest exposure was achieved in the stomach followed by the small intestine and adrenal gland. Pharmacokinetic studies were done by using Sprague-Dawley (SD) rats, oral absorption was fast and plasma exposure was dose-dependent and oral bioavailability were low. At the same dose, FHND6081 exposure was severalfold higher in whole blood than in plasma, which was consistent with blood cell partitioning. Moreover, only a small fraction of the parent compound was excreted via feces and urine and oxidative metabolites were detected in feces and plasma.Conclusion: The overall preclinical pharmacokinetic profile supported the selection and development of FHND6091 as a clinical candidate.